The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.
A spark ignition direction injection (SIDI) engine may be operated in two engine modes: a homogeneous (i.e., stoichiometric) mode and a stratified (i.e., lean) mode. The homogeneous mode is similar to a conventional spark ignition mode. An air/fuel mixture is introduced into a combustion chamber during an intake stroke of an engine cycle. The air/fuel ratio is approximately 14.7 for gasoline engines. The cylinder charge is nearly homogeneous in composition, temperature, and residual level when an intake valve is closed.
In the stratified mode, the engine is operated at a diluted air/fuel mixture that is lean of stoichiometry (for example, the air/fuel ratio may be approximately 20-30). The fuel is injected to a region near a spark plug at a later stage of the compression stroke. The engine is operated in a more fuel efficient manner during the stratified mode. When medium or high load and high speed are desired, the engine is operated in the homogeneous mode. When low load and speed are desired, the engine is operated in the stratified mode. Engine torque fluctuations may occur when the engine is transitioned between the homogeneous mode and the stratified mode.